Naphthalenedicarboxylic acids and the diesters of naphthalenedicarboxylic acids are useful chemical intermediates. Naphthalenedicarboxylic acids and diesters of naphthalenedicarboxylic acids, for example, are monomers that can be used to prepare high performance polymeric materials such as polyesters. These naphthalenedicarboxylic acids include 1,2-, 1,3-, 1,4-, 1,5-, 1,6-, 1,7-, 1,8-, 2,3-, 2,6 - and 2,7-naphthalenedicarboxylic acid.
2,6-Naphthalenedicarboxylic acid (2,6-NDA) and diesters of 2,6-naphthalenedicarboxylic acid, particularly dimethyl-2,6-naphthalenedicarboxylate (DM-2,6-NDC), can be used to prepare poly(ethylene-2,6-naphthalate) (PEN) by the reaction of 2,6-NDA or a diester of 2,6-NDA with ethylene glycol. Fibers and films made from PEN polyester have improved strength and thermal properties relative to other polyester materials. In addition, films made from PEN demonstrate superior resistance to gas diffusion and particularly to the diffusion of carbon dioxide, oxygen and water vapor. Because of its exceptional properties, PEN is especially useful in applications such as food and beverage containers (particularly in so-called "hot fill" food and beverage containers), tire cord and magnetic recording tape.
Although PEN can be prepared from both 2,6-NDA and DM-2,6-NDC, in some instances it is advantageous to prepare PEN from DM-2,6-NDC. For example, a polyester manufacturer may only have equipment available for storing liquid monomer feed materials and/or for adding liquid feed materials to a polymerization reactor. DM-2,6-NDC would be suitable for this operation whereas the acid, 2,6-NDA, would not. This is because pure DM-2,6-NDC has a melting point of approximately 190.degree. C. and, consequently, can be heated to above its melting point and maintained there in the liquid or molten form. In contrast, 2,6-NDA is a solid that undergoes decomposition at temperatures greater than 300.degree. C. and, therefore, cannot be utilized as a liquid. Additionally, some polyester manufacturers may only have polymerization processes for preparing polyester materials from ester monomers. These manufacturers would necessarily require DM-2,6-NDC, the diester, rather than 2,6-NDA.
Monomer purity is an additional factor that must be considered when deciding whether to manufacture polyesters from diester monomers or diacid monomers. While 2,6-NDA, for example, may be readily obtained by the oxidation of a suitable feedstock such as 2,6-dimethy-, 2,6-diisopropyl- or 2-acetyl-6-methylnaphthalene by one or more oxidation processes, the 2,6-naphthalenedicarboxylic acid obtained therefrom is generally of insufficient purity to be used directly for preparing PEN. Furthermore, 2,6-NDA is extremely difficult to purify. As mentioned above, it does not melt and cannot, therefore, be purified by distillation. It is also essentially insoluble in most common solvents making purification by standard recrystallization processes unsuitable. In contrast, DM-2,6-NDC is a liquid at high temperatures and can be distilled to achieve purity levels required for manufacturing high quality PEN polyester.
Although solid at normal temperatures and pressures it is sometimes preferable to transport and store DM-2,6-NDC as well as the other naphthalenedicarboxylic acid diesters in a molten state. Liquids are easily transferred between storage containers and, as mentioned above, many polyester manufacturers add monomer feeds to polymerization reactors as liquids and consequently prefer to have the monomers supplied as liquids.
While maintaining naphthalenedicarboxylic acid diesters, and particularly DM-2,6-NDC, in the molten state is at times convenient for shipping and for use during the manufacture of PEN or other polymers, the molten diesters degrade at the temperatures used during storage. Although the exact mechanism for the degradation is not known, factors such as the reaction of the hot diester with the oxygen in air, thermal chemical reactions and reaction with ambient moisture, cause a reduction in the quality of the diesters maintained in the molten state. Degradation is undoubtedly accelerated at the high temperatures used for maintaining these diesters and particularly DM-2,6-NDC in the molten state. Additionally, most industrial storage and shipping vessels are constructed of steel. The exposed steel surface in these containers may also promote the degradation of the naphthalenedicarboxylic acid diesters.
The degradation of molten diesters of naphthalenedicarboxylic acids that occurs during high temperature storage results in the discoloration of the ester, an increase in the concentration of acidic materials and in the formation of various impurities. This degradation, particularly the increase in color and the increase in the concentration of impurities that affect the subsequent polymerization step, severely reduces the commercial usefulness of the naphthalenedicarboxylic acid diesters.
One possible method for eliminating or reducing the degradation caused by maintaining the diesters at elevated temperatures is to use molten diesters having one or more stabilizer compounds contained therein. Many stabilizers, for example, anti-oxidants, are commercially available and are used in a variety of applications. These stabilizer compounds include diarylamines such as diphenylamine and di-octyl-diphenylamine, diaryldiamines such as N,N'-diphenyl-p-phenylenediamine and N,N'-di-betanaphthyl-p-phenylenediamine; phenolics such as 2,6-di-t-butylphenol, catechol and pyrogallol; sulfur containing compounds such as dilauryl thiodipropionate; phosphorus-containing compounds such as phosphines, phosphates and phosphites; metal salts of sulfur and phosphorus compounds such as zinc dithiophosphates, as well as a variety of other materials. In particular, certain liquid monomer materials have been stabilized in order to prevent degradation during storage. For example, styrene, a widely used monomer for preparing polystyrene, is stabilized with 4-t-butylcatechol. Molten maleic anhydride may be stabilized with 4,4'-di(hydroxyphenyl) alkanes or with 4-alkylphenols as is disclosed in U.S. Pat. No. 4,062,874 to Sciaraffa and Cermak or, alternatively, with trialkyl trithiophosphites as is taught in U.S. Pat. No. 3,998,854 to Samans and Spatz. Molten dimethylterephthalate may be stabilized with a variety of stabilizer compounds such as a mixture of hindered phenols and dialkyl phosphites as disclosed in U.S. Pat. No. 3,445,504 to Mehalso; ethylene glycol as disclosed in 3,485,867 to Jackson; low molecular weight monohydric saturated alcohols as is disclosed in U.S. Pat. No. 3,505,390 to Hoffmann; catechol, pyrogallol, quinone, hydroquinone, t-butyl catechol, butylated hydroxytoluene, phenol, toluhydroquinone, triphenylphosphite, Primene JMT (t-(C.sub.18-22)-alkyl amines) and Primene 81-R (t-(C.sub.12-14)-alkyl amines) as is disclosed in U.S. Pat. No. 3,659,007 to Giambra; bis-(beta-hydroxyethyl) terephthalate as is disclosed in U.S. Pat. No. 3,742,026 to Mori et al., alkali metal salts or alkoxides plus phosphites as is disclosed in U.S. Pat. No. 4,058,663 to Black and U.S. Pat. No. 3,461,153 to Tholstrup and Rush. None of these disclosures, however, teaches the stabilization of the diesters of naphthalenedicarboxylic acids and, in particular, the stabilization of dimethyl-2,6-naphthalenedicarboxylate.
It would be desirable to have a composition comprising a diester of a naphthalenedicarboxylic acid that does not undergo excessive degradation when stored or used at elevated temperatures. It would also be desirable to have a method for stabilizing liquid esters of naphthalenedicarboxylic acids, and particularly molten dimethyl-2,6-naphthalenedicarboxylate, during shipment or storage at high temperatures. The present invention provides such compositions and methods.